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We humans have been around for hundreds of thousands of years, and for most of that time, food has been hard to get. We've had a relatively stable food supply (thanks to agriculture) for only about 10,000 years.

Now lots of people have weird relationships with food. For example, did you know that if you are offered varying amounts of food on a plate, you'll end up eating more if there's more food on the plate? This can happen regardless of how hungry you are.

Welcome to the famous portion size effect.

In one typical study, people were offered a macaroni-and-cheese dish for their lunch. It came in various weights — 500, 625, 750 and 1,000 grams. People ate 30 per cent more food if they were offered the 1,000 gram portion as compared to 500 grams. The portion size effect is also clearly seen at the movies. People stuff themselves more, when their portion of popcorn or candy is larger — regardless of how hungry they actually are.

The way we eat in the western world has changed a lot over the last few generations. People eat food outside the family home more and more. Take-away food is convenient, relatively cheap, highly palatable and (you guessed it) in large portions.

In fact, over the last few decades, the portion sizes of food have increased across the board — in supermarkets, restaurants and fast food establishments.

Strangely, most of us are unaware of what is an "appropriate" portion of food — in other words, how much we should eat to provide us with all the energy and nutrients that we need. Instead, we usually eat either a constant volume or a constant weight of food. This is independent of whether the food is a low-energy density soup, or something really loaded with calories or kilojoules, such as peanut butter, or a hamburger with cheese and bacon.

There's a whole bunch of irrational factors going on with the portion size effect.

First, most of us are not really consciously aware of the size of the food portions that we eat. Mind you, we do think that we are getting better value-for-money if we get the larger portions.

Second, we eat more when the same size portion is served in a larger bowl than when it is served in a smaller bowl.

Third, if we are eating ice-cream, we will eat more if we use a larger spoon than if we use a smaller spoon. So it seems that we humans use various visual shortcuts when we eat. Following on from that, if we then underestimate large portion sizes, and also do not try to work out how much energy we need to eat, then we are likely to find ourselves susceptible to over-eating. Suddenly, we are eating more than we need.

But this portion size effect is poorly understood. Sure, sensory influences (such as what you see or smell) are important. But what about cognitive factors, such as what you learnt from your parents, the norms of the bigger society around you, financial considerations, and so on?

And what about after you've eaten the meal, the so-called post-ingestive consequences? Now here's a few weird findings. First, the more you eat, the less pleasant and enjoyable each mouthful becomes. And second, whether you eat a big portion or a little portion, your feelings of hunger and fullness are the same at the end of eating.

But consciously understanding that the portion size effect exists, helps us deal with this evolutionary hangover from our distant past, ie, to stuff ourselves because food was usually scarce.

We can serve ourselves smaller portions when eating at home. In a restaurant, we can ask for a smaller portion, or if we get a portion that is too large, we can simply divide it in half, and eat only one half.

Another way to take advantage of the portion size effect is to remember that it applies to foods that some people find not particularly palatable, such as vegetables (although, personally, I do love my veggies, especially a slightly browned and crispy eggplant). So serving kids large portions of veggies might trick them into more of what's good for them, which is a good thing — because you can't always teach good taste.